Why are particles are missing, when using Volume/Grid Emission?
One way to solve this is to bevel each vertex of the cube very little. Tab into edit mode. A to select all vertices. Press Ctrl+V and select "Bevel". Then give it a very small amount of Bevel Offset (I used 0.001 here). Then Tab out of edit mode and it'll work.



Another simpler way to do it is to select all the faces and hit Ctrl+T to triangulate the faces.